Abstract Hematological malignancies that include leukemia and lymphoma are often treated with allogeneic hematopoietic stem cell transplantation (allo-HCT). However, chronic graft-versus-host disease (cGVHD) remains a prominent cause of transplant-related morbidity and mortality despite available immunosuppressive regimens. Few prophylactic strategies have been successful at reducing the incidence of cGVHD in patients after allo-HCT. An area previously unexplored as a treatment for cGVHD involves the unfolded protein response (UPR). Three master regulators control the UPR: PERK, IRE-1?, and ATF6. When IRE-1? becomes activated, its primary function is to splice Xbox binding protein-1 (XBP-1u) mRNA. Spliced XBP-1 (XBP-1s) mRNA is translated into XBP-1 protein which acts as an effective nuclear transcription factor. Immune responses such as B-cell proliferation and antibody production require large amounts of properly folded proteins. As a transcription factor, XBP-1 protein relieves ER stress by up regulating cellular machinery responsible for protein folding and degradation. XBP-1 is therefore required for the effector function and survival of various types of immune cells that are susceptible to ER stress. IRE-1?/XBP-1 signaling axis plays predominate roles in B cells and dendritic cells (DCs) among other immune cells. Built upon published findings and our preliminary observations, we will evaluate how IRE-1?/XBP-1 signaling axis impacts in the development of cGVHD after allo-HCT through regulating B cells and DCs among others. Our Central Hypothesis is that XBP-1 plays an essential role for B-cell and DC activation and function, and targeting XBP-1 will restrain allogeneic responses leading to the control of cGVHD while preserving the integrity of cytotoxic T lymphocytes (CTL) and thus maintaining the graft-versus-leukemia (GVL) effect. This hypothesis will be tested in the following two Specific Aims: 1) Define the contribution of XBP-1 on hematopoietic cells in the development of cGVHD after allo-HCT using a genetic approach; 2) Determine the therapeutic effect of targeting XBP-1 in the control of cGVHD and leukemia relapse using a pharmacological approach. The current study is expected to further understand the cell biology how UPR regulates immune responses, reveal the role for IRE-1?/XBP-1 signaling axis in the development of cGVHD and relatable hematologic malignancies, and provide a novel therapy for controlling cGVHD and leukemia relapse in after allo-HCT.